1. Field of the Invention
The present invention is broadly concerned with clothing items such as footwear and apparel which can be inductively heated for cold weather use. More particularly, the invention is concerned with such clothing items and methods of use thereof wherein the items include an induction heatable element which is heated when subjected to an alternating magnetic field. The invention also pertains to assemblies including such heatable clothing along with an induction heater designed to heat the elements of the clothing. In preferred forms, the clothing items include a device serving to limit the maximum temperature of the heatable elements, and closed loop, wireless temperature feedback allowing temperature control and maintenance.
2. Description of the Prior Art
Heated clothing such as footwear or apparel has a number of advantages, particularly for those who work outside in cold climates or for those engaged in cold-weather sports such as skiing. Such heated clothing can improve physical performance, minimize cold-related discomfort, and can provide a degree of safety during prolonged winter time exposure.
Many methods for heating clothing have been proposed in the past. The two most common techniques utilized either battery power or chemical energy. Battery powered heatable clothing items include relatively heavy batteries and associated resistance heating circuitry. Such systems can be a problem because the circuit wiring may be broken during extended use and can be difficult to launder. Moreover, the batteries tend to be bulky and are often placed in awkward positions, such as on the wrist for heated gloves. Chemical energy systems use chemical packs that heat when exposed to oxygen. The user places these packs inside pockets of apparel or, in the case of footwear, as inserts placed adjacent the soles of the footwear. These heating packs do not perform well where airflow is restricted, such as in footwear applications. Further, these packs are designed for single use only, which significantly increases costs and creates waste disposal problems.
U.S. Pat. Nos. 5,956,866 and 5,140,131 describe battery/resistance heating systems in footwear and other clothing items. Similarly, U.S. Pat. Nos. 6,620,621 and 5,977,517 describe battery powered heatable apparel. The '621 patent specifically discloses battery-warmed gloves requiring a battery on each glove. The '517 patent employs heatable panels placed inside a vest, and powered by a battery. U.S. Pat. No. 6,148,545 uses an external heater applied to footwear. This patent also suggests use of phase change material to store heat produced by the external device. This system does not permit reheating while the footwear is worn, and requires long warming times owing to restricted heat transfer over small surface areas. All of these systems suffer from the problems of excess weight, lack of durability and cleanability issues.
U.S. Pat. No. 6,701,639 describes a removable shoe insole heated by an exothermic chemical reaction. In this system, the user must remove the footwear and the associated insole in order to insert the heating source. Again, this type of heating is deficient in that the heating elements are of single use design and must be periodically replaced by the user.
There is accordingly a need in the art for improved heatable clothing which does not add significant weight or complexity to the clothing, which can be readily reheated without removal of the clothing, and which provides closed loop temperature feedback control during heating.